A tank-venting system of the kind which is of interest here has basically the following assembly groups: a tank; an adsorption filter which is connected to the tank via a tank connection line; the adsorption filter further having a venting line which can be closed by a venting check valve; and, a tank-venting valve in a valve line which connects the adsorption filter to the intake pipe of the corresponding internal combustion engine.
At the present time, the most important class of methods for checking the tightness of a tank-venting system is based on the generation of a difference pressure in the system. This difference pressure can either be an overpressure relative to the ambient pressure or it can be an underpressure. In order to generate an underpressure, the venting line on the adsorption filter is closed and the tank-venting valve is opened. In this way, the tank-venting system can be evacuated with the aid of the underpressure in the intake pipe. In the case of generating an overpressure, the venting line is likewise closed and, with the aid of a blower, air is forced into the system. Typically, a difference pressure of approximately 10 hPa is generated.
A soon as the desired test difference pressure is reached, the tank-venting system can be closed entirely and the value of the decay gradient of the built-up difference pressure is determined. If the system is tight, then the difference pressure decays only very slowly; otherwise, the decay is relatively rapid. A conclusion can therefore be drawn as to the condition of tightness when the determined value is less than a threshold decay gradient for the difference pressure.
It is obvious that the decay gradient for the difference pressure is not only influenced by unwanted inflowing air but also by vapor which vaporizes from the fuel in the tank. Such vaporization occurs almost always when the contents of the tank move with intensity. For this reason, known methods provide that the above-described tightness check is only then carried out when a condition is satisfied which makes it probable that the test result is not falsified by the vaporizing fuel. In the simplest case, the check comprises that in advance of the closure of the tank-venting valve, a check is made as to whether the mixture control had to carry out a lean correction in the time span with the tank-venting valve open. This is then the case when air enriched with fuel vapor inflows from the tank-venting system. More reliable statements are obtained with a complex test condition which not only inquires as to the just-mentioned lean correction but also checks whether the motor vehicle is moved so that the contents of the tank are probably likewise moved. For this purpose, an inquiry can be made as to whether the motor vehicle, on which the tank-venting system is mounted, is at standstill. Additionally, a check can be made as to whether idle is present. Corresponding methods are, for example, described in U.S. patent application Ser. No. 08/070,334, filed May 26, 1993, and incorporated herein by reference.
The conventional methods, which provide a conclusion as to the tightness of the tank-venting system with the aid of a difference pressure decay gradient, have the following general steps in common: making a check as to whether a condition, which can include several subsidiary conditions, is satisfied, which permits a reliable tightness check to be expected; when this condition is satisfied, generating a difference pressure (overpressure or underpressure) in the system and closing the system when a pregiven difference pressure is reached; determining the value of the decay gradient of the built-up difference pressure; and, drawing a conclusion as to tightness of the system when the determined value is less than a threshold decay gradient.
The known arrangements are configured to carry out such a method.
It is apparent that a check with the aid of the above-mentioned decay gradient is that much more precise the greater the time span and pressure range are over which this decay gradient is determined. It would therefore be ideal to set relatively high difference pressures of, for example, 100 hPa. The selection of a high difference pressure and a long test time are, however, contrary to various aspects. A first aspect is that in the case of adjusting underpressure, the fuel vaporizes with increasing intensity which, as explained above, falsifies the test result. A second aspect is that the fuel tank is sensitive to pressure and is especially sensitive in the underpressure range. A third aspect and the most important one listed here is the test time. Attention is here called to the fact that the above-mentioned conditions, for which especially reliable test results can be expected, occur only infrequently and then not for a time span which is very long. In conventional methods, the entire time span for the build-up of the difference pressure and for the determination-of the decay gradient should amount to not more than a few ten-second intervals, for example, not more than 30 to 40 seconds.
The conventional methods use relatively low difference pressures in order to operate with total test time spans of this kind. For this purpose, a value of approximately 10 hPa was already mentioned.